Binary search. Just need to convert index.

1 class Solution { 2 public: 3     bool searchMatrix(vector
     
      
        > &matrix, int target) { 4         if (matrix.size() == 0) return false; 5         int n = matrix.size(), m = matrix[0].size(), start = 0, end = n*m-1, mid = 0; 6         while (start <= end) { 7             mid = (start + end)/2; 8             if (matrix[mid/m][mid%m] == target) return true; 9             if (matrix[mid/m][mid%m] > target) end = mid-1;10             else start = mid+1;11         }12         return false;13     }14 };